Now the question is "what's the limit?". The answer is very
surprising: You may bin the color components up to 8:8 with acceptable
effects on the final image!

Following is the demonstration:

Let's start from a known color image (some flowers in my garden)

By bringing the color saturation to zero, the black & white
(L) image is extracted

R G B components are extracted from the original image and
reduced times 8
(yes, that's the actual size!)

By resizing x8 the above R G B images and combining them we
obtain the RGB image (without L), not very good as can be expected...

Using LRGB processing, this is the reconstructed image, very
close to the original. You will notice the "painting" effect on the
overexposed area of the top flower.

As a conclusion, when color imaging deep sky, use the maximum
available binning from the CCD camera for the R G B images. Also, applying a
small smoothing filter on the R G B images should not hurt as it is better to
loose on color definition than to bring noise from the R G B components.